ON THE RELATION BETWEEN THE ISRA AND THE EM ALGORITHM FOR POSITRON EMISSION TOMOGRAPHY

The image space reconstruction algorithm (ISRA) was proposed as a modi fication of the expectation maximization (EM) algorithm based on physi cal considerations for application in volume emission computed tomogra phy. As a consequence of this modification, ISRA searches for least sq uares solutions instead of maximizing Poisson likelihoods as the EM al gorithm. In this paper, we show that both algorithms may be obtained f rom a common mathematical framework. We use this fact to extend ISRA f or penalized likelihood estimates.